1. Field of the Invention
The present invention relates to a microstructured optical fibre.
Furthermore, the present invention relates to an optical communication line and an optical communication system comprising said microstructured optical fibre and to a method and a preform for making the same.
2. Description of the Related Art
As known, the optical fibres are commonly used in the field of optical telecommunications for the transmission of signals. Essentially they typically comprise an inner cylindrical region, called core, within which an optical signal is, transmitted and an outer annular region, called cladding. The cladding has a refractive index lower than that of the core in order to confine the signal transmitted within the latter.
Typically, the core and the cladding are made from a silica based vitreous material and the difference in the refractive index between the core and the cladding is obtained by incorporating suitable additives (dopants) in the vitreous matrix of the core and/or of the cladding.
A microstructured optical fibre is an optical fibre having small structures such as, for example, air holes running through the fibre lengthwise.
Three different types of microstructured fibres—corresponding to three different principles of propagation—have been distinguished and described in the prior art (see for example, D. J. Richardson et al., “Holey fibers—A review of recent developments in theory, fabrication and experiment”, ECOC 2000, Vol. 4, page 37).
In the first type of fibre (hereafter called microstructured clad fibre or MCF) the core and the cladding are made of the same homogeneous background material (e.g., silica). Guided propagation in the core is achieved by providing the cladding with microstructures (e.g., air holes) having a refractive index lower than the refractive index of the background material.
In the second type of fibre (hereafter called photonic bandgap fibre or photonic crystal fibre), the core and the cladding are made of the same homogeneous background material (e.g., silica) and the cladding is provided with microstructures (e.g., air holes) arranged according to an appropriate periodic array. The periodic array forms in the cladding a photonic bad-gap at the propagation wavelength so that light is confined in the core thanks to total reflection at the core/cladding interface.
In the third type of microstructured fibre (hereafter called microstructure assisted fibre, or MAF), the core and the cladding are made of different background materials (e.g., doped silica/silica) so that guided propagation in the core is achieved mainly thanks to a refractive index difference between the core and the cladding background materials, just as in a conventional fibre. The cladding is provided with microstructures having the role of tailoring the optical properties of the fibre.
The present invention relates to this third type of microstructure assisted fibres.
Throughout the present description and claims, the expression                “microstructure” is used to indicate a structure (e.g., a hole, a column, a ring, a cellular ring structure) which is disposed in a background material of a microstructured optical fibre, has a refractive index different from the refractive index of the background material and runs through the fibre lengthwise;        “microstructured region” is used to indicate a region of a microstructured optical fibre wherein microstructures are provided;        “background material” is used to indicate the material of a microstructured region wherein the microstructures are disposed; that is the material of the interstitial spaces between the microstructures of a microstructured region;        “chromatic dispersion coefficient D” is used to indicate the first order dependency of the group velocity from the wavelength. In particular, the chromatic dispersion coefficient D is expressed as follows        
  D  =                    ⅆ                  β          1                            ⅆ        λ              =                  -                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            c                                λ            2                              ⁢              β        2            where β1 and β2 are the constant of propagation of the mode of the first and, respectively, of the second order, λ is the wavelength and D is expressed in ps/(nm*Km);                “slope of chromatic dispersion S” or “slope S”, unless otherwise stated, is used to indicate the derivative, with respect to the wavelength, of the chromatic dispersion coefficient D and is expressed in ps/(nm2*Km);        “transmission optical fibre” is used to indicate an optical fibre used in a line or in an optical communication system for the transmission of optical signals from one point to another one located at an appreciable distance (for example, of at least some km or tenths of km).        
Takemi Hasegawa et al., (“Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss below 1 dB/Km”, OFC 2001, paper PD5-1) disclose a hole assisted lightguide fibre (HALF) wherein the role of the holes is to depress the effective refractive index around the core so as to form equivalent W-shaped structure for chromatic dispersion compensation. The HALF disclosed has a pure silica core, a fluorine-doped silica cladding and four holes that directly surround the core. The diameter of the core and holes is 10 μm and 5 μm, respectively.
WO 01/98819 discloses a microstructured fibre including a core region, a moat region including a ring of air columns, and a cladding region, surrounding the core and moat regions, which comprises either a periodic lattice of columns formed from a solid material or a solid ring. The core region is formed from a high index material and the moat region is formed from a material having a refractive index lower than the refractive index of the core region. The cladding region is formed from a material having a refractive index which is higher than the refractive index of the moat region and lower than the refractive index of the core region. Furthermore, the columns of the moat region contact the outer circumference of the column structure forming the core region. The distance between the centres of adjacent columns is not mentioned in this document.
The Applicant observed that in order to obtain a smooth and effective changing effect of the refractive index in the microstructured region of a microstructured fibre, a high number of microstructures is required. That is, structures closely spaced and small in diameter are desirable.
Furthermore, the Applicant noted that a high number of microstructures is required also to improve the stability of the optical fibre performance with respect to imperfect positioning and/or size of the microstructures.
On the other hand, the Applicant observed that a small number of microstructures is desirable for other technological problems. For example, it is desirable for making the production process of a microstructured fibre easier, for improving the strength of the fibre, for improving fibre splicing and for reducing the fibre attenuation. As to the fibre attenuation, the Applicant remarks—in fact—that in a microstructured fibre the existence of a free glass surface in the air holes may be a source for water pollution during the fibre preform drawing process and an additional source of Rayleigh scattering from the roughness of the surface.
Accordingly, the Applicant faced the technical problem of providing a microstructure assisted optical fibre that allows to achieve a good compromise between the above mentioned conflicting requirements.